I don't own the 135, but I do own and use the 100-300 D lens, this is well made and light enough to be a good match for the NEX. Importantly my tests and images consistently show it to out perform my 28-135 at 135mm. It is far less flare prone too.

The added advantage is that it is also a fine lens at all lenghts up to about 230mm or so and would provide you perhaps with a bit more flexibility.

If you have not seen one, you may be surprised just how compact it is. Oh and just in case your wondering, yes it is better than the 70-210 and 100-200 by a significant margin, according to my experience.

Having said all this, I have seen some great results from the 135mm 2.8 and it is a surprisingly small lens, but if you were going down the pathway if wanting a compact and sharp manual focus lens in this focal lenght, both the nikon 135mm f3.5 and the similar olympus versions are very good and cheap.